Such a digital-to-analog (DA) converter is known under the type designation PNA 7518 and operates in accordance with the known parallel selection system. Herein the reference voltage divider divides the reference voltage into a plurality of equal steps which are available at the nodal points which are connected to the analog output via the switching transistors. Depending on the digital input signal, the decoder circuit activates one of the switching transistors, as a result of which the value of the analog output signal corresponds to the voltage across the relevant nodal point.
This type of DA converter is of a simple structure since always only one switching transistor needs to be activated. However, the signal quality of the analog output signal is reduced by signal distortion as a result of a signal level-dependent RC-time constant of the charging circuit by means of which the parasitic capacitance at the analog signal output is charged and also by switching noise caused by turning on and off of the switching transistors and by the current surges with which the parasitic capacitance is charged. The signal quality reduction limits the effective bit-resolution and/or the processing rate of this prior-art DA-converter.
The parasitic capacitance is always charged or discharged to a different analog signal voltage via a series resistor the effective value of which depends on the position of the selected nodal point on the reference voltage divider. The rate at which the voltage at the analog output can change its value is not the same for all the signal levels. The characteristic staircase (shaped) noise signal at the analog output signal consequently includes components having frequencies which are located in the useful band of the analog output signal which cannot be filtered off by means of the customary low-pass filter. These components manifest themselves as a distortion of the analog output signal which is relatively ever increasing versus an increasing bit-resolution or versus an increasing processing rate of this type of DA-converter.
A first cause of the signal level-dependent series resistance in the charging circuit is the ever changing effective output resistance in the nodal points of the reference voltage divider. In the center, the output resistance is at its maximum, at the ends it is at its minimum. A second cause is the varying forward resistance between the first and the second main electrode of the switching transistors. The selection signals for the control electrodes of all the switching transistors are all of the same value. However, the voltage between the control electrode and the first main electrode, which is decisive for the forward resistance of the switching transistor, varies because the first main electrodes are connected to different taps of the reference voltage divider.